Update documentation (README)

[g0di]

The documentation (code examples and outputs) are not aligned with latest version of the code and is also a bit messy for users:

• Remove the « instance: null » which is now omitted from problem details responses
• Mention that « null » values are stripped from output problem json
• Mention the fact that problem details title is inherited from the status code when not provided as well as the detail property (see http.HTTPStatus enum)
• Enhance the doc about how to build a problem details object (response object, exception object), passing headers and custom properties
• Add a section mentioning other existing similar projects and highlight differences

[g0di]

Missing so far:

• comparison with other libraries
• library technical API details